DRAFT/Discovery: render elements to 2d canvas

[dvoytenko]

Summary

Partial for #3803

A quick exploration for 2d rendering for editor elements. The main goal is the contrast detection, but could also be useful for page thumbnail generation.

There's not much unexpected here. The only smallish caveats:

1. Everything should always be untainted. We should get this for free for fonts and for all media coming from media library.
2. We always have to wait for media and fonts to load before rendering. So, the rendering is a single-critical-path queue.
3. Rendering of wrapped/centered/tokenized HTML with correct line height is will be a headache. So it depends on how "perfect" we need the rendered canvas to be comparing to HTML version. For contrast detection and even thumbnails, arguably, we need only a good approximate quality.

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).